This invention relates to a corrosion resistant iron-chromium-nickel-molybdenum alloy suitable for use in its wrought form, and to a method of producing a wrought alloy body from said alloy.
More specifically, this invention relates to a wrought analog of a type CA-6NM alloy casting (ASTM specification A-743 and A744) and a method for producing a wrought body by rolling at elevated temperature.
Type CA-6NM cast stainless steel has a nominal composition by weight of 12% chromium, 4% nickel and 0.4 to 1.0% molybdenum, a maximum carbon content of 0.06%, and the remainder substantially iron. ASTM specifications recite the following chemistry for CA-6NM:
______________________________________ ELEMENT WEIGHT % ______________________________________ Cr 11.5-14.0 Ni 3.5-4.5 C 0.06 max Mn 1.00 max Si 1.00 max Mo 0.40-1.00 P 0.04 max S 0.04 max Fe + residuals remainder ______________________________________
Type CA-6NM alloy is hardenable by heat treatment, exhibits good corrosion and cavitation resistance properties and is used in many applications including castings for the hydroelectric industry, pump castings, valve bodies, compressor impellers, diffuser impellers, turbine blades, steam and gas turbine casings and ship propellers.
Type CA-6NM alloy was developed primarily in response to the low impact strength of CA-15 (ASTM specification A-351). For example, the Charpy V-notch impact strength of the CA-15 alloy is about 15 ft. lbs. at 32.degree. F. whereas the minimum impact strength of the CA-6NM alloy is about 30 ft. lbs. with actual values of about 65 ft. lbs. at 32.degree. F.
CA-6NM also shows improved casting behavior over the CA-15 alloy with a lower tendency toward cracking during and after solidification of heavy sections, and therefore requires less need for repair welding.
Historically, standard CA-6NM chemistries have been somewhat unsuitable for hot working due to the relatively low elongations and reduction of area percentages. Heretofore, CA-6NM type alloy bodies have been used only in their cast and heat treated states. Consequently, none of the advantages imparted by hot working an alloy body to form a wrought product are present in commercially available CA-6NM alloy bodies.
As is known generally, hot working imparts to an alloy body beneficial properties which cannot be obtained satisfactorily by other techniques. For example, hot working provides controlled directional physical properties to the alloy body, resulting in significant advantages in engineering designs that account for differences in transverse and longitudinal properties. Previously, castings had to be reinforced in areas of greater stress in order to obtain greater strength and resistance to cracking to compensate for homogeneity in physical properties. A body formed from the wrought alloy of the present invention inherently is stronger in the direction of the major hot working thereby simplifying certain design considerations. Furthermore, the yield, tensile and impact strength of the alloy body are increased by hot working.